This application claims priority of European Patent Application No. 98305165.7, which was filed on Jun. 30, 1998.
This invention relates to a method and apparatus for inserting pilot symbols into communication signals, and in particular to a protocol for inserting staggered pilot symbols into a synchronous multiple user Code Division Multiple Access (CDMA) signals.
In radio communication systems, multiplexing schemes are employed to share common frequency bandwidth between multiple users. This enables one terminal to communicate with another over a radio channel using a coded spread spectrum signal with codes which are almost uncorrelated, thereby providing a multiple access system in which the collisions between signals broadcast concurrently are not destructive. In CDMA systems, the signal from each of N different users is multiplexed by substituting each bit within the signal by a number of shorter bits, called xe2x80x9cchipsxe2x80x9d, chosen according to a particular xe2x80x9cchannelization codexe2x80x9d. Since the occupied bandwidth expands after the substitution, this process is often called xe2x80x9cspreadingxe2x80x9d and CDMA systems are often referred to as spread spectrum systems. After spreading, the signal from each user occupies the total available bandwidth. The signal is detected and decoded at a receiver by means of correlation with the appropriate channelization code.
In multi-user CDMA systems all signals transmitted from the same transmitter may be made synchronous, i.e. all bits, and therefore all chips, are aligned in time as in, e.g., the base station transmitter for a downlink communication, i.e. base station to mobile station communication. This allows for the use of orthogonal channelization codes which prevent multi-user interference, although some cross-talk due to multipath propagation may still occur. The multipath propagation can be accounted for by xe2x80x9crakexe2x80x9d-detection at the receiver. This means that the receiver applies several detection xe2x80x9cfingersxe2x80x9d, with each finger detecting a particular multipath component. The finger outputs are then combined to finally form the improved received signal. For simplicity of the description we assume a single path channel and hence do apply a xe2x80x9csingle fingerxe2x80x9d receiver rather than a full rake receiver. The invention works for a rake receiver accordingly.
Coherent detection is used to improve the detection and decoding process at the receiver. In order to facilitate the coherent detection process the receiver needs to estimate the current channel conditions which may be time varying and which may also depend upon the propagation environment. Typically, pilot symbols, whose data information is known to the receiver, are inserted into the signal at the transmitter. For example, one continuous pilot signal, using a dedicated xe2x80x9cpilot channelization codexe2x80x9d. Alternatively, the transmitter may insert pilot symbols periodically, using the individual channelization codes of each multiplexed signal; thus each signal has its own time-multiplexed pilot symbols, inserted in between data symbols. At the receiver, changes in the channel conditions are interpolated using appropriate algorithms. However, these interpolation algorithms introduce inaccuracies in the channel estimation process which affect the data detection and decoding processes.
There is a requirement for a flexible and improved detection process which avoids introducing inaccuracies in particular through an improved channel estimation.
According to a first aspect of the invention there is provided a method of inserting pilot symbols into communication signals comprising:
generating a plurality of signals to be transmitted, each signal being divided into frames, each frame consisting of a plurality of blocks;
generating a plurality of pilot signal blocks to be inserted into the signals before transmission; inserting into each frame of each signal a pilot signal block; and characterized in that
the pilot signal blocks are inserted in a time staggered fashion, spread though out the frame.
A receiver receiving a transmitted signal may make use of the pilot signal blocks in the other transmitted signals.
According to a second aspect of the invention there is provided a method of inserting pilot symbols into communication signals comprising:
generating a plurality of signals to be transmitted as a composite signal, with each signal divided into frames, each frame consisting of a plurality of blocks;
for each signal, inserting a pilot signal block into each frame before transmission; characterized in that
the pilot signal block of each signal is inserted so that the pilot signal blocks within the composite signal are time staggered though the frame.
A receiver receiving a transmitted signal may use the pilot signal blocks which are present in the composite signals.
The pilot symbols are time-staggered using an arbitrary algorithm for the pilot symbol insertion. This enables each receiver to perform more accurate, continuous channel estimation and results in a reduced Bit Error Rate (BER) and an improved Quality of Service (QoS).
In a conventional system an arbitrary user k (for example: a mobile user, receiving a composite multi-user CDMA signal, with the users being synchronous) has its own time-multiplexed pilot symbols (i.e. a block of pilot symbols). The receiver for user k estimates the channel by using its dedicated pilot symbols and conventionally, the pilot symbols are time-aligned. When the pilot symbols remained time-aligned among all users the receiver may only obtain only one channel estimate per frame.
A receiver according to the invention makes use of pilot symbols of all other users in the system in order to improve its channel estimation for user k. This gives more reliable data detection and for this the receiver needs to know the channelization codes of all other users, and the positions of the respective pilot symbol blocks relative to the frame timing. It is assumed that the receivers can obtain this information from the various control channels and by knowing the pilot symbol insertion algorithm (that is, how the pilot symbol blocks are arranged). It is advantageous to time-stagger the pilot symbols among the users, the receiver can then take estimates of the time-varying mobile communication channel more often and, thus, it becomes easier to track the channel by means of interpolation or any other channel estimation algorithm. However, due to the time-varying nature of the mobile radio channel, for a good channel estimate tracking performance it is better to take several channel estimates (at different time instances) during one frame rather than taking only one at the beginning of each frame; the two algorithms presented below provide such preferred pilot symbol block-arrangements among all users within a frame.